The Definition of the Taxon Tetrapoda

There is currently a debate concerning the definition of the taxon
Tetrapoda, which is part of a wider debate dealing with the definiton
of phylogenetic groups. Many papers over recent years (eg. Gauthier
et al., 1989, Rowe, 1988, Rowe et al., 1992, de Queiroz et al., 1992)
have suggested that a phylogenetic, or node-based, definition, is more
rigorous than one which employs a "key-character", with both
a "crown-group" definition (Altangerel et al., 1993, Norell
and Novacek, 1992, Norell et al., 1993, Lebedev and Coates 1995) and
a "total-group" definition (Patterson, 1993, Coates, 1996)
having its adherents. In the case of the Tetrapoda, the crown-group
would encompass the clade containing all the modern representatives,
namely the Amphibia and the Amniota, plus any fossil taxa falling above
the node in the cladogram representing their nearest common ancestor.
In that case, Tetrapoda would exclude any fossil taxon which falls outside
this grouping, independent of the possession of any key character such
as limbs with digits. It has been stressed elsewhere (Clack and Coates,
1995) that as a key character, "limbs with digits" is unsatisfactory
since it can be broken down into many separate characters. The "total-group"
definition would include not only all members of the crown group but
also its complete stem-lineage back to a node defined by the nearest
common ancestor of the crown group and its closest living sister-group
(in the case of tetrapods, that would be either lungfishes or coelacanths).

There are some major practical problems with node-based definitions.
First, almost inevitably, by the "crown-group" definition,
many fossil taxa which indisputably have limbs with digits are excluded.
Thus Acanthostega and Ichthyostega
would not belong within the Tetrapoda, since they are stem tetrapod
plesions outside the crown group. A strict application of the crown-group
method not only forbids the use of the term "Tetrapoda" for
these animals, but disqualifies them as "tetrapods". Conversely,
by the "total-group" definition Tetrapoda would include many
animals which clearly do not have limbs with digits, not only "fish"
such as Eusthenopteron and Panderichthys
but the rest of the osteolepiform lineage.

Both node-based definitions are equally rigorous, defensible and valid
phylogenetic definitions of the group Tetrapoda but in both cases, the
result is counterintuitive in respect of some animals. The problem arises
not with node-based definitions per se, but with the logical difficulty
of applying to a node-defined group a name whose origins are etymologically
founded upon an innovative "key-character" as so many higher
taxonomic group names are. Lee and Spencer (1997) have pointed out other
problems concerned with stability of node-based groupings.

Either way in this case, the origin of limbs and the origin of terrestriality
are distanced from the origin of tetrapods, which may be an advantage
in that it separates our perceptions of these three independent evolutionary
strands.